Tracking issue: GAT improvements

[LukasKalbertodt]

This issue collects all major parts of the library that can be improved with generic associated types. This library won't be released as 1.x before GATs have landed and these improvements have been implemented, as they a central to a fast and convenient API.

• Element iterators: Explicit{Edge, Face, Vertex}::{edges, faces, vertices}() currently return Box<Iterator>. Instead, those traits should have a GAT {Edge, Face, Vertex}Iter<'s> instead which is returned by the methods. -> Try replacing next_handle_from based iteration #32
• PropMap: the trait currently uses a hack from the boo module. -> done
• IntoMeshWriter could also move its parameters to the function and an GAT